Photovoltaic devices are commonly used to convert light energy into electrical energy. These devices contain an active layer which is of a light absorbing material that generates charge carriers upon exposure to light. A typical example of such a material is mono- (m-Si) and poly- (p-Si) silicon. Since silicone is expensive, it is important to keep the layer thickness to a minimum. In the case of m-Si and p-Si, the layer thickness is relatively thick because the material is also used as a substrate for creating the photovoltaic device. The total layer thickness of the silicon is therefore around 180 μm. To overcome this and other problems thin film photovoltaic devices were developed. Said devices use another material (e.g. glass, plastic or metal foil) as a substrate and only a thin layer (±0.1-4 μm) of the active material is applied to this substrate. Examples of light absorbing materials which are used in thin photovoltaic applications are amorphous silicon (a-Si), microcrystalline silicon (μc-Si), copper indium gallium selenide (CIGS), cadmium telluride (CdTe) and dye-sensitized solar cell (DSC).
A light absorbing material does not absorb all wavelength of light equally efficient. The percentage of photons at a given wavelength that generate charge carriers is called the quantum efficiency (QE). Each light absorbing material has a different QE. In some cases therefore several layers of different light absorbing materials are applied on top of each other to absorb as much light as possible. For example a combination of two light absorbing materials may be used of which one has a high QE in the blue/green region of the light spectrum and another the other one the peen/red region of the light spectrum. By doing so a broader range of wavelengths is effectively absorbed and converted into charge carriers. Atypical example is using a combination of a-Si and μc-Si.
A thin film photovoltaic device contains, besides the active layer, several other components. For example it is necessary to collect the charge carvers that are created when light is absorbed by the active layer. Hereto electrodes are placed on the front (light receiving side) and back (non-light receiving side) of said active layer. Especially the front side TCO is required to have a high conductivity (sheet resistance typically 8-14 Ohm/square) while also showing a high transmittance. Otherwise the active layer would not receive any light and would thus not be able to generate charge carriers. Usually a transparent conductive oxide (TCO) is used for this purpose. Examples of TCO materials are indium tin oxide (ITO), fluorinated tin oxide (FTO) or (aluminum) zinc oxide (AZO). The back electrode is often based on a highly reflective material such as silver. The reason for this is that light which is not absorbed by the active layer is reflected back into the active layer by the reflective electrode. Hence, the path length of the incident light is increased and thus also the chance of absorption. For the purpose of increasing the path length of light it is also possible to use a transparent back electrode with a separate reflective layer (such as a white foil) behind it. In any case, the stack of thin films is very thin and can be easily damaged. To protect them, a transparent cover made of glass or polymers is used on the front side (i.e. the light receiving side). The back can be protected by both transparent or non-transparent materials such as glass, tedlar epoxy resin etc. The back and front cover is often laminated together by using ethylene vinyl acetate (EVA), ionomers, thermoplastic polyurethanes (TPU) or polyvinylbutyral (PVB).
Thin film modules are manufactured by depositing the thin layers (i.e. electrodes and active layer) via techniques as chemical or physical vapor deposition on a substrate. In principle various materials can act as a substrate, however, commonly the protective cover plate is used for this purpose. For example if one starts with a glass cover plate first the TCO is applied, next the a-Si layer and the silver electrode are deposited and finally the back is sealed by a protective polymeric coating.
For thin film applications it is very important to keep the layer thickness as thin as possible, but without loss of efficiency of the device. A thinner layer results in less material costs and faster processing. On the other hand, it is important that the layer is thick enough to absorb most of the incoming light. Light which is not absorbed cannot be converted into electrical energy which results in a poor efficiency of the photovoltaic device. In the prior art several methods are known to overcome this problem.
A common method to reduce the layer thickness of the active material is to create a texture, which refracts light, into the front electrode or TCO. As a result of the refraction of light the path length of the light into the active material is increased. The relief texture can be a random texture (J. Müiler et al., TCO and light trapping in silicon thin film solar cells, Solar Energy, Vol 77, Issue 6, December 2004 (917-930)). Control over the refraction of light is however limited and the increase in path length is therefore small. It is also possible to create a periodic texture (C. Haase et al., Efficient light trapping scheme by periodic and quasi-random light trapping structures, Photovoltaics Specialists Conference, 2008) into the TCO. A periodic texture can better control the refraction of light, however the manufacturing is expensive since multiple complicated processing steps are required. An alternative method is to make a texture which diffracts light, such as a grating, into the TCO (C. Haase, H. Stiebig, Light trapping in thin-film silicon solar cells with periodic structures, Proc. 21st European Photovoltaic Solar Energy Conference and Exhibition, Dresden, Germany, 2006, p.1712). Such elements are capable of redirecting light quite effectively, however, only for a specific (range of) wavelength. Regardless of the type of texture, there are several disadvantages of using the concept of a textured TCO. Texturing introduces defects into the TCO which causes absorption of free carriers and thus loss of efficiency of the photovoltaic device. In addition a TCO is not fully transparent and does absorb some of the incident light. To texture a TCO it is necessary to use a thicker layer of TCO material, which does result in more absorption of light. Consequently less light can reach the light absorbing active layer and can thus be converted into charge carriers. This also reduces the efficiency of a thin film photovoltaic device.
Since it is not always possible to directly texture the TCO, it is also possible to indirectly texture the TCO by texturing the substrate on which the TCO is applied. For example the glass front cover can be textured before applying the TCO (U.S. Pat. No. 6,538,195B1). By subsequently depositing the TCO on the textured glass, the texture of the glass is also present in the TCO. Regardless of the method and way of texturing the principle disadvantages are similar to the once discussed in the previous paragraph. In addition the textured glass is not compatible with the laser patterning processes through the glass.
In some cases a thin film photovoltaic device is manufactured by depositing the reflective back electrode, light absorbing active layer and the TCO on a substrate which is used at the back cover rather than the (glass) front cover. For these devices it is, analogous to the above-discussed possibilities, possible to texture the reflective back electrode or the substrate on which the reflective back electrode is deposited (U.S. 2009/194150 A1). Also this method of texturing suffers from the already discussed disadvantages.
EP 0 991 129 A1 describes a photovoltaic module comprising a textured glass cover plate. The texture renders anti-glaring properties to the cover plate. Furthermore, the document proposes the use of an index-matching agent for enabling a laser patterning of the thin films from the light incident side through the glass by optically compensating the effect of the front texture. EP 0 991 129 A1 mentions the TCO properties in example 1 as Sn02, film thickness of 700 nm and a peak-to-peak roughness of 200 nm.
U.S. 2002/129850 A1 proposes the use of an anti-glare layer, preferably made of an organic binder material containing light scattering particles. Both the composition of the layer and the surface roughness are responsible for the layer's anti-glaring properties. Since such a layer can be applied to a finished photovoltaic module, such a layer can be applied after laser patterning, thus solving the compatibility problem of having a light scattering cover and the requirement of laser scribing through the front cover
U.S. 2008/115828 proposes the use of textured cover glass comprising non-textured edges in order to render it compatible with the state of the art encapsulation process of framed modules. U.S. 2008/115828 mentions ZnO or SnO2 as transparent conductive materials suitable for photovoltaic modules. The textured layer renders anti-reflective properties to the plate.
From the above it can be concluded that methods to increase the path length of light into the active material of a thin film photovoltaic device are either non-effective or involve high manufacturing costs. It could be said that in essence the problem here is that non-optical elements (i.e. electrodes) are used as optical components for enhancing the path length of light into the light-absorbing layer. It is therefore an objective to overcome these problems.